Body worn physiological sensor device having a disposable electrode module

ABSTRACT

A method for providing high voltage circuit protection for a patient monitor. The method includes providing a substrate that supports one or more electrical connections to a patient&#39;s body. The method further includes determining a print pattern and thickness of a first material having a first resistivity to be printed on the substrate, determining a print pattern and thickness of a second material having a second resistivity to be printed on the substrate, printing the first material onto the substrate, and printing the second material onto the substrate wherein at least part of the second the material overlays the first material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of, and claims thepriority and benefit to, U.S. patent application Ser. No. 11/591,619,entitled “Body Worn Physiological Sensor Device Having a DisposableElectrode Module,” filed Nov. 1, 2006, the entirety of which is herebyincorporated by reference.

FIELD OF THE INVENTION

This invention relates generally to a physiological monitor and moreparticularly to a body worn physiological monitor.

BACKGROUND OF THE INVENTION

Measurements of various physiological parameters are important to thestudy of the human condition. Physiological measurements can beparticularly important in a health care setting, such as in a hospital.One of the more important physiological measurements performed on apatient is the electrocardiogram (ECG), showing the condition of thehuman heart.

Portable patient monitors have evolved that allow patients to enjoy atleast some mobility. Typically a battery operated monitor can be hung ona belt, shoulder strap, or carried by a patient using some other similarhanging arrangement. Sensors, such as ECG electrodes, are affixed to thepatient's body, such as with tape, and connected to the battery operatedmonitor by wires. After a fixed interval of time, or at a low batteryindication, the batteries can be replaced or recharged. One example of aportable patient monitor is the Micropaq wireless patient monitor,manufactured by Welch Allyn, Inc., that permits multi-parametermonitoring and patient alarm capabilities built in a small, rugged,lightweight, patient-wearable device.

Another version of a portable physiological monitor is the heart ratemonitor typically used by individuals engaged in an athletic activity.The monitor includes a sensor, which generally makes direct or indirectcontact with an individual's chest to monitor heart beats and then bywires, or by wireless techniques, the sensor transmits the sensed heartbeat to a nearby microcomputer based monitor and display. Such unitsgenerally measure only heart beat and are not capable of doing any ofthe traditional ECG analysis functions.

A recurrent problem with the portable monitors typically used inhealthcare applications is the need for wires from sensors situated onthe patient's body to the portable unit. These wires can become tangledand cause discomfort or become unplugged when inadvertently pulled ortugged on. In addition, wire motion can increase ECG noise due to thetriboelectric effect. Muscle movement can also increase ECG noise, dueto the typical placement of ECG electrodes over major muscles. Moreover,portable monitor battery maintenance (e.g. battery recharging orreplacement) can be time consuming and costly.

Another problem is related to the requirement that a medical grademonitor survive multiple defibrillation cycles of at least 360 joules.Conventionally, this requirement has been met by one or more powerresistors situated in series with the wire leads of a fixed or portablephysiological monitor. The problem is that the physical volume ofconventional power resistors is too large for use in a compact monitorapplication.

Another shortcoming of small sensor devices is that these devices lackthe intelligence to vary the amount and type of data transmitted,depending on patient condition. Exercise heart monitors do not transmita full patient waveform for clinical analysis while medical monitorsmeasure and transmit the full patient waveform, even when the patient ishealthy. While transmitting the full patient waveform is the preferredsolution from a purely clinical standpoint, such transmission requiressignificant power to transmit large amounts of data and restricts thedesign from being small and inexpensive.

Yet another problem is that arrhythmia analysis is a computationallyintensive operation not well-suited to existing small portable monitorsthat presently have no ability to perform arrhythmia analysis.

Therefore, there is a need for a body worn combined physiological sensorand monitor having a disposable sensor, but used and worn by a patientas a single unit directly and non-permanently affixed to a patient'sbody. Also, what is needed is a physically compact resistive element forprotecting a body worn device from damage caused by multipledefibrillation cycles. Also, what is needed is a medical-grade monitorthat can intelligently measure and transmit data only as required toalert clinicians that the patient needs additional attention. What isalso needed is a body-worn device capable of running arrhythmia analysisthrough computationally efficient algorithms.

SUMMARY OF THE INVENTION

According to one aspect, a body worn patient monitoring device comprisesat least one disposable module including a plurality of electricalconnections to the body. The electrical connections are coupled to askin surface of the patient to measure physiological signals of thepatient. The at least one disposable module includes a disposable moduleconnector. The body worn patient monitoring device includes at least oneinternal or external power source to power the body worn patientmonitoring device. The body worn patient monitoring device also includesat least one communication-computation module, having acommunication-computation module connector to receive physiologicalsignals from the at least one disposable module via said disposablemodule connector. The communication-computation module also includes atleast one microprocessor to actively monitor the patient and to performa real-time physiological analysis of the physiological signals and aradio circuit to communicate a raw physiological signal or a result ofthe physiological analysis at a predetermined time or on the occurrenceof a predetermined event, via a radio transmission to a remote radioreceiver, wherein the at least one disposable module is mechanically andelectrically coupled directly to the at least onecommunication-computation module. The body worn patient monitoringdevice, including the at least one disposable module and the at leastone communication-computation module, is directly non-permanentlyaffixed to the skin surface of the patient.

According to another aspect, a method of providing high voltage circuitprotection for a body worn monitor comprises the steps of: providing asubstrate that supports one or more electrical connections to apatient's body; determining a print pattern and thickness of a firstmaterial having a first resistivity to be printed on the substrate;determining a print pattern and thickness of a second material having asecond resistivity to be printed on the substrate; printing the firstmaterial onto the substrate; and printing the second material onto thesubstrate wherein at least part of the second material overlays thefirst material.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of these and objects of the invention,reference will be made to the following Detailed Description which is tobe read in connection with the accompanying drawings, in which:

FIG. 1A shows an exemplary body worn physiological monitor having adisposable electrode module;

FIG. 1B shows a partially unassembled side view of the body wornphysiological monitor of FIG. 1A;

FIG. 1C shows an assembled side view of the body worn physiologicalmonitor of FIG. 1A;

FIG. 1D shows a bottom view of the body worn physiological monitor ofFIG. 1A;

FIG. 2 shows an exploded perspective view of an exemplary body wornphysiological monitor;

FIG. 3 shows an exploded perspective view of an exemplary computationand communication module;

FIG. 4A shows an exemplary disposable unit flexible circuit board;

FIG. 4B shows a partial enlarged view of a portion of the flexiblecircuit board of FIG. 4A, further showing an exemplary resistive tracehaving a fillet;

FIG. 4C shows a partial side elevated view of a portion of the flexiblecircuit board of FIG. 4A, further showing an exemplary conductivesurface overlaying a resistive material;

FIG. 4D shows a partial side elevational view of the circuit board ofFIG. 4A, further showing an exemplary conductive surface having a snapreceptacle;

FIG. 5 shows a block diagram of one embodiment of a body wornphysiological monitor having a power source in a disposable unit;

FIG. 6 shows a block diagram of one embodiment of a body wornphysiological monitor having a power source in or connected to thecomputation and communication module;

FIG. 7A shows a schematic diagram of a direct connected referenceelectrode used in conjunction with a body worn physiological monitor;

FIG. 7B shows a schematic diagram of a virtual reference electrode asused in conjunction with a body worn physiological monitor;

FIG. 8 shows a schematic diagram depicting one embodiment of an analogswitching arrangement provided on a body worn physiological monitor toselect a reference electrode configuration;

FIG. 8A shows an exemplary driven lead circuit topology for use withelectrodes of a body worn physiological monitor;

FIG. 9 shows an exemplary ESIS filter circuit topology with circuitprotection;

FIG. 10 shows an alternative circuit protection to that depicted in FIG.9;

FIG. 11 shows a flow chart for an algorithm utilized by a body wornphysiological monitor to detect whether a patient has a pace maker;

FIG. 12 shows a circuit topology of a high pass filter useful forbaseline restoration;

FIG. 13 shows seven graphs of amplitudes plotted versus frequency toillustrate exemplary operation of the circuit of FIG. 12;

FIG. 14 shows two exemplary positions for a body worn ECG monitor to benon-permanently affixed directly to a patient's body;

FIG. 15 shows a block diagram of a setup for simulating the effect ofpatient defibrillation on resistive traces;

FIG. 16A symbolically shows resistive dots silk screened on a tray;

FIG. 16B shows a histogram of an exemplary resistive distribution ofbaked resistive dots; and

FIG. 17 shows an exemplary ECG waveform.

Package styling varies slightly between the drawings. Such minordifferences, e.g. the case styling of computation and communicationmodule 102, illustrate minor variations in mechanical packaging suitablefor use as body worn monitors. Drawings are not necessarily shown toscale.

DETAILED DESCRIPTION

A “body worn” device is described herein with regard to certainexemplary embodiments. A “body worn” device is defined herein as adevice that is directly, but non-permanently, affixed to a patient'sbody. A “body worn monitor” is a device that can be directly “worn” onthe patient's body as a single unit, including one or more physiologicalsensors and a communications and computation module to perform at leastinitial processing of one or more physiological measurements made usingone or more physiological sensors. Unlike prior art patient-wearabledevices, at least one sensor can be incorporated into the device thatmakes a direct or indirect (such as by capacitive coupling) electricalconnection with the patient's body without the use of external wiresfrom sensors to the device. In addition and unlike athletic heartmonitors, a “body worn” monitor can be a full functioning medical grademonitor, e.g. meeting the requirements of European Unions' MedicalDevice Directive and other applicable industry standards, such as EC-13for an electrocardiograph. The body worn medical-grade monitor caninclude a device, for example, such as a pulse oximeter, CO₂ monitor,respiration monitor, or can function as an ECG monitor, incorporatingphysiological sensors, front end analog electronic signal conditioningcircuits, and a microcomputer based computation unit with wirelessreporting of measured physiological data, all contained within in a“body worn” package that can be non-permanently affixed directly to apatient's body. A body-worn medical-grade monitor can also includeadditional measurement capabilities beyond those mentioned here.

FIGS. 1A-1D depict various views of an exemplary body worn physiologicalmonitor 100 having a communication and computation module 102 and adisposable electrode module 110. In this exemplary embodiment,physiological monitor 100 is designed for use as an electrocardiogram(ECG) monitor for obtaining and recording and/or transmitting ECGinformation, including ECG waveforms and alarms for a person, such as apatient, wearing body worn physiological monitor 100.

FIG. 1A shows an exemplary top view of body worn physiological monitor100. A crescent shape allows the body worn physiological monitor 100 tobe placed on the chest of a patient, and more specifically around thepectoralis major, to allow measurement of lead configuration I, II, orIII, or to be placed on the patient's side allowing measurement using aV-lead configuration. Though not shown, multiple body worn physiologicalmonitor 100 units can be used to effectively provide multiple leads. Byplacing electrodes around the body so that they are not situateddirectly atop major muscles, both motion noise artifacts and musclenoise artifacts can be prevented. Moreover, by eliminating cables, noisedue to cable motion or compression (i.e. triboelectric effect) can beeliminated.

FIG. 1B shows a side elevated view of physiological monitor 100 havingan exemplary attachment mechanism, such as a retention clip 104, tomechanically attach communications and computation module 102 to the topsurface of the disposable electrode module 110. Flexible printed circuitlayer 101 can be made from a thin insulating material, such as accordingto this embodiment, a 75 micron thick layer of Mylar®. Typicallyelectrical traces (not shown in FIGS. 1A-1D) on flexible printed circuitlayer 101 can be further protected by an insulating covering, analogousto a conformal coating. A formed plastic layer or a cloth with adhesiveon one side thereof can be used to cover and protect flexible printedcircuit layer 101, as well as to provide an aesthetic outer layer tomake the body worn monitor 100 visually appealing.

FIG. 1C shows a side view of physiological monitor 100 in which thecommunications and computation module 102 has been affixed to disposableelectrode module 110. FIG. 1D depicts a view of the underside ofexemplary physiological monitor 100 showing one embodiment of disposableelectrode module 110 having electrodes 109. In this embodiment, eachelectrode 109 comprises electrode gel 103 and conductive surface 404(FIG. 4). Together, electrode gel 103 and conductive surface 404 createa half cell, such as, for example, a Silver/Silver Chloride half cell.Also and according to this embodiment, conductive surface 404 candirectly accept the electrode gel 103.

FIG. 2 shows an exploded assembly view of the exemplary body wornphysiological monitor 100. As noted with regard to FIGS. 1A-1D, the bodyworn physiological monitor 100 includes a removable and reusablecommunications and computation module 102 and a disposable electrodemodule 110, the later including electrode gels 103 for ECG monitoringand batteries 204 to power communications and computation module 102. Aflat planar insulating/adhesive member 105 includes a plurality ofopenings that are each sized to receive electrode gels 103. Theinsulating member 105 provides a bottom side cover for the flexibleprinted circuit layer 101 and augments adhesion to the human body.Electrode gel 103, when attached to an appropriate substrate such assilver-silver-chloride or other substrate, can be used to establish arelatively low impedance electrical connection between conductivesurface 404 (FIG. 4) and the patient's skin.

Electrode gels 103 can adhere to a patient's skin. While electrode gel103 is typically an adhesive electrode gel, the adhesion offered byelectrode gels 103 alone might not give a sufficient holding force fornon-permanently affixing body worn physiological monitor 100 to apatient. To achieve a better adhesion of body worn monitor 100 to apatient's skin, insulating/adhesive member 105 can be used tonon-permanently affix body worn physiological monitor 100 to a patient.Thus, body worn monitor 100 can be applied to a patient in the same wayan adhesive strip is applied, such as for example, those adhesive stripssold under the brand name “BAND-AID®”. One exemplary type of foamadhesive suitable for affixing a flexible circuit board to a patient is1.6 mm adhesive foam from Scapa Medical of Bedfordshire, UK. As shown inFIGS. 1B and 1C (although not to scale), each of the electrode gels 103extend sufficiently below adhesive layer 105 in order to ensure goodelectrical connection with a patient's skin surface (not shown). Tab106, FIG. 1A, allows for easy removal of a protective backing 111 fromadhesive layer 105.

Flexible printed circuit layer 101 can include contacts, such as batteryclips (not shown), to receive and connect to batteries 204. (It iscontemplated that in some future embodiments, a single battery canprovide sufficient electrical power.) In the exemplary embodiment, asshown in FIG. 2, batteries 204 can be mounted under respective batteryflaps 205 arranged on opposite sides of the flexible printed circuitlayer 101. Alternatively, battery clips (not shown) or battery holders(not shown) can be used to provide both mechanical support andelectrical connections for each of the batteries 204. One type ofbattery holder suitable for such use, for example, is the model 2990battery holder, manufactured by the Keystone Electronics Corp. ofAstoria, N.Y. Battery cover 107 provides protection for batteries 204 aswell as a surface to press upon when applying electrodes 103 toconductive surface 404. Retention clips 104 can comprise, for example, aplurality of spring fingers with latching clips. Retention clip 104,affixed to disposable electrode module 110, can be used to securereusable communications and computation module 102 to disposable package110. Reusable communications and computation module 102 is hereinillustrated in a simplified representation, including cover 201,communications and computation printed circuit board assembly 202, andbase 203.

FIG. 3 shows a mechanical view of an exemplary reusable communicationsand computation module 102, as well as a preferred method for making anelectrical connection between flexible printed circuit layer 101 indisposable electrical module 110 and communications and computationprinted circuit board assembly 202 situated in reusable communicationsand computation module 102. In this exemplary embodiment, communicationsand computation printed circuit board assembly 202 can include aplurality of press fit and/or soldered conductive sockets 301 forreceiving electrical plug 302, the plug having a corresponding pluralityof conductive pins. Each conductive pin shown in plug 302 can correspondto an electrical connection pad on flexible printed circuit layer 101. Arow of mechanical sockets 303 can receive the multi-pin row of plug 302.Thus, an electrical connection can be made between each pad of flexibleprinted circuit layer 101 having a conductive post on plug 302 and eachcorresponding conductive socket 301 on communications and computationprinted circuit board assembly 202. Posts 304 can align and secure eachof the cover 201, communications and computation printed circuit boardassembly 202, and base 203. Note that in FIG. 3, a simplified drawing ofcover 201 omits slots to receive retention clip 104 to affixcommunications and computation module 102 to disposable package 110. Abody worn monitor 100 would typically also include retention clip 104FIG. 2, or other suitable type of mechanical clip(s), in order toprovide a secure mechanical connection between communications andcomputation module 102 and disposable electrode module 110.

FIG. 4A shows one embodiment of flexible circuit board 101 in anexpanded (e.g. unassembled) view. Flexible circuit board 101 is formedon a substrate 406. Substrate 406 can be cut, for example, from a Mylarsheet of suitable thickness. In this embodiment, one battery 204 can bemounted adjacent to a conductive surface 404. Conductive gel 103 (notshown in FIG. 4) can be mounted on the exposed conductive side of aconductive surface 404.

Conductive surface 404 can also be viewed as the electrode portion of ahalf cell and electrode gel 103 can be considered to be the electrolyteportion of a half cell. In conventional terms of art, the combination ofelectrode and electrolyte and ECG electrode is typically referred to asa half cell. For example, the combination of a conductive surface 404and an electrolyte layer (e.g., electrode gel 103) forms a half cell.For convenient quick reference to a half cell structure, the term“electrode” (assigned reference designator “109”) is usedinterchangeably with “half cell” herein. It is understood that intypical embodiments, electrode 109 comprises conductive surface 404 andelectrode gel 103.

Typically, electrodes make use of a circular or square conductivesurface. Increasing the ratio of the perimeter of the surface to thearea of the surface affects current density distribution anddefibrillation recovery.

For convenience, we define the term “annulus” herein and throughout asthe region between two simple curves. A simple curve is a closed curvethat does not cross itself. Under this definition, an annulus caninclude substantially square shapes, substantially rectangular shapes,substantially circular shapes, substantially oval shapes, as well assubstantially rectangular shapes with rounded corners. Further weinclude in the definition of annulus, the case of a substantially “U”shaped surface as described by a single closed curve.

One exemplary electrode gel 103 suitable for such use on a body wornmonitor is type LT00063 hydrogel supplied by Tyco Healthcare of Prague,Czech Republic. Typically, a conductive surface 404 creates theelectrode portion of the half cell. By increasing the ratio of perimeterto area of the circular electrode portion of the half cell, the signalto noise ratio of the input ECG signal can be increased.

As depicted herein on the exemplary circuit layout, two batteries 304can be connected in series, with one polarity being made available atconnection pad 407 from battery connection 402, battery connection 401creating the series connection between the two batteries, and connectionpad 410 providing the second polarity of the series connected batteries.Note that in some embodiments, a single battery alternatively may beused in lieu of the exemplary arrangement or two batteries can be alsowired in parallel, depending on the voltage requirements of a particularcommunications and computation module 102.

Connection pads 408 and 409 electrically couple the signals fromelectrode gels 103 (not shown in FIGS. 4A-4D) via conductive surface 404and resistive traces 412 to electrical plug 302 (not shown in FIGS.4A-4D). Electrode contact pad 405 can be connected via resistive trace413 to connection pad 411 to provide a direct-connected referenceelectrode (not shown in FIGS. 4A-4D). Traces 412 extending betweenconductive surface 404 and connection pads 408 and 409 and trace 413extending between conductive surface 405 and connection pad 411 can bemade from resistive materials including resistive metals, carbon, silverink, powders, paints, or other material of determinable electricalresistance.

Resistive traces on flexible circuit board layer 101 replace the bulkypower resistors needed by prior art monitors, having electrodes orsensors connected by wires or leads. These resistive traces shouldsurvive multiple defibrillation cycles such that body worn monitor 100remains functional even after one or more attempts to re-start apatient's heart. In order to survive defibrillation, the resistivetraces should dissipate that portion of the potentially damagingdefibrillation energy that is coupled into the monitor. This fractionalportion of the defibrillation energy typically enters body worn monitor100 from electrodes 109, FIG. 1D (electrodes 109 including conductivesurface 404 and electrode gel 103).

It is desirable that the resistances of the protective resistive tracesbe in a range between about 1 kilo ohm to about 10 kilo ohms. Below 1kilo ohm, depending on the resistive material used, it can be morelikely that the resistance of the resistive traces 412 and 413 willincrease with each successive defibrillation pulse. Above 10 kilo ohms,a high resistance impairs the signal to noise ratio, specifically due tothermal noise, which has a mean square value of 4*k*T*R*BW, where “k” isBoltzmann's constant, “T” is temperature measured in degrees Kelvin, “R”is resistance in ohms, and “BW” is bandwidth, in Hz, which becomessignificant relative to the EC-13 requirement that the noise referred toinput be less than 30 μV peak-to-valley.

Power dissipation in the herein described traces can be calculated byE²/R, in which E refers to the potential across the trace and R is theresistance of the trace. R can be calculated by ρ*L/A, where ρ is theresistivity of the material used to form the trace, L is the length ofthe trace, and A is the cross-sectional area of the trace.

In developing resistive traces for use on a flexible printed circuitlayer 101, typically formed on a Mylar substrate 406, such as shown inFIG. 4A, various materials were tested. Silver, including silver inks,while useable, was found to be less desirable, because it was difficultto achieve sufficiently thin silver traces on Mylar to achieve highenough resistances. Carbon, including carbon pastes and carbon inks, wasalso tried and found to be suitable. In order to use carbon however,several additional problems had to be solved. At 1 kilo ohm, the powerdissipated by the resistors caused them to degrade across multipledefibrillation cycles. The solution was to make carbon traces in therange of about 8 to 10 kilo ohms. 10 kilo ohm resistances proved to be agood compromise between noise levels, power dissipation by theresistors, and manufacturing tolerances for depositing carbon ink on aMylar substrate to dissipate the power from multiple defibrillationcycles. To achieve the desired resistance of about 10 kilo ohms(interchangeably represented herein as “10 k” or “10 kΩ”), for a givenresistivity of the carbon paste, and given trace width and thickness(height), the length of the trace is then defined. In some cases, suchas for traces 412, the length needed for a trace run, as betweenconductive surface 404 and connection point 409, might be longer thanthe length defined for a particular trace resistance (typically 10 k).In this case, traces can be extended by lengths of silver conductivetraces. There can be a short overlay distance, on the order of 5 to 10mm, in which a silver trace overlaps the carbon trace to provide a morerobust connection between the resistive and conductive portions of thetraces. Where overlap is used, the overall length of the resistiveportion can be adjusted slightly to maintain the desired overallresistance.

Another problem associated with carbon traces was arcing at theinterface between the carbon and conductive traces. Arcing wasparticularly problematic at the abrupt connection between the carbontrace and conductive surface 404. Arcing was also observed to occurbetween the end section of the carbon trace and conductive surface 404.(Electrode gels 103 create the conductive path to the patient throughconductive surface 404 and a layer of conductive gel.)

According to one solution to the above noted arcing problem, as shown inFIG. 4B, a rounded (fillet) section 430 of carbon trace can be added atthe interface to conductive surface 404. A fillet or “tear drop” shapecauses the carbon trace to become gradually wider as it connects toconductive surface 404 and relieves the electrical potential stress atthe interface.

An alternative solution to the arcing problem is shown in FIG. 4C,wherein carbon can be laid down beyond the trace (412) to include apattern of conductive surface 404 formed from carbon. A carbon annuluspattern can be deposited before the conductive surface 404 is deposited.Conductive surface 404 can then be deposited as an overlay over theearlier formed carbon annulus shape. Finally, conductive gel 103 can beattached to the conductive surface layer (the carbon layer residingbetween conductive surface 404 and the Mylar substrate used as flexiblecircuit board 101). Both of the aforementioned arcing solutions can beused together. It should also be noted that conductive surface 404 canbe formed from suitable materials other than silver, including, forexample, materials such as silver chloride.

Arcing can also occur between the resistive traces and other (typicallysilver) conductive traces on the flexible circuit board 101. Trace totrace arcing can be suppressed by allowing sufficient spacing betweenthe traces. Generally a minimum spacing of about 3 mm/kV, as required byASNI/AAMI DF80:2003 57.10 BB, has been found to be sufficient to preventtrace to trace arcing from a defibrillation event. Closer trace spacing,as close as 0.01 mm/kV, can be employed successfully by first applyingan insulating dielectric layer, similar to a conformal coating, over thesurface of flexible circuit board 101 that covers most of the substrateand traces. The insulating dielectric layer can be prevented fromforming or adhering to conductive surface 404, such as by use of a maskduring application of the insulating layer.

In an alternate embodiment, as depicted in FIG. 4D, a snap device can beadded to conductive surface 404 to accept a manufacture snap-onelectrode (not shown), such as, for example, the ConMed Cleartrace lineof ECG electrodes including the model 1700 Cleartrace electrodemanufactured by the ConMed Corp. of Utica, N.Y. or similar typeelectrodes made by the 3M Corp. of St. Paul, Minn. When designed toaccept a snap-on electrode, the conductive surface 404 is typicallysmaller than in the previous embodiment. A receptacle snap 432 forreceiving the commercial snap-on electrode can be inserted by anysuitable method, such as by press fitting or other fastening method,into conductive surface 404, typically also penetrating throughsubstrate 406. In this embodiment, arcing can be similarly suppressed inthis embodiment by adding a fillet to the carbon-conductive surfaceinterface and/or deposing a conductive surface 404 over a carbon layeras previously described.

Example: Resistive traces and an annulus were tested on a substrateformed from CT3 heat stabilized treated polyester (75 microns thick),such as manufactured by the MacDermid Autotype Corp. of Schaumburg, Ill.Resistive traces were silk screened onto the substrate using 7102 carbonpaste conductor from the DuPont Corporation of Wilmington, Del. Thecarbon paste conductor was deposited through a 43T silk screen mesh. Thesubstrate containing the paste deposit was then cured inside a fanassisted air circulated oven at 120° C. for a period of 5 minutes. Thetraces formed were about 55 mm long and 2 mm wide, having an overallthickness of about 7.5 microns. The initial measured resistance of eachtrace was about 14 kilo ohms. After each trace was subjected to 3defibrillation cycles, the measured resistance increased to about 15kilo ohms. Over a 3 mm length, the trace widens to about 5 mm,terminating into a carbon annulus with an outer diameter of about 20 mmand an inner diameter of about 10 mm. A silver layer of PF-410 silverink from the Norcote Corp. of Eastleigh Hampshire, UK was then depositedover the carbon annulus, also to an overall thickness of about 7.5microns. The deposition of the silver layer was via the silk screenprinting method, in which a 90T silk screen mesh was used. The substratecontaining the deposited silver ink was then cured inside a fan assistedair circulated oven at 120° C. for a period of 15 minutes. A thirddielectric insulating layer comprising SD2460, components A & B(dielectric and hardener), manufactured by Lackwerke Peters GmbH+Co KGof Kempen, Germany, and having a thickness of approximately 13 micronswas then deposited over the traces and substrate, but not over theannulus. (The electrodes were formed by attaching a conductive gel tothe annulus. The conductive gel used was LT00063 hydrogel from TycoHealthcare of Prague, Czech Republic.) Again, the silk screen printingprocess was used to deposit the dielectric layer through a 90T screenmesh. The substrate was placed again into a fan assisted air circulatedoven at 120° C. for a period of 30 minutes.

Example: Silver traces for use as conductive (not resistive) traces on abody worn monitor circuit substrate were formed from a silver paste thatwas silk screened onto a Mylar substrate. 45 mm long traces had ameasured resistance in a range of 3.5 to 6 ohms, 75 mm traces had ameasured resistance in a range of 6.5 to 13 ohms, and 105 mm traces hada measured resistance in a range of 10 to 16 ohms. The deposition of thesilver layer was performed via the silk screen printing method in whicha 90T silk screen mesh was used. The substrate containing the depositedsilver ink was then cured inside a fan assisted air circulated oven at120° C. for a period of 15 minutes.

FIG. 15 diagrammatically depicts an exemplary test setup used tosimulate the effect of a patient defibrillation on resistive traces.Defibrillator 1501 was used to apply multiple defibrillation shocks of360 Joules each to the 100 ohm resistor 1502. The 100 ohm resistoraccording to this setup simulated a patient's body. Note that most ofthe defibrillation energy goes into the patient's body by design, torestart the patient's heart. Resistive traces 1503 and 1504 were wiredacross resistor 1502, also as shown in FIG. 15, in order to simulate theelectrical circuit that would be formed between the resistive traces ina body worn monitor situated on a patient undergoing defibrillation.Neon bulbs 1506 were used as part of the protection circuitry that canbe used with resistive traces in a body worn monitor. 400 ohm safetyresistor 1505 was present as a precaution to limit short circuit currentin the event of a test setup failure. Both the 100 ohm resistor(simulating human skin resistance) and the 400 ohm safety resistor wereused in accordance with medical specification AAMI EC-13. Following 3defibrillation shocks of 360 Joules each, the measured resistance of the10 k carbon track changed from 10 k to 11 k following the first shock,to 13.1 k following the second shock, and to 13.2 k following the thirdshock. The measured resistance of the 9.7 k trace changed from 9.7 k to11.6 k following the first shock, to 13.0 k following the second shock,and finally to 13.2 k, following the third shock. In a subsequentrelated test, the 100 ohm resistor was replaced by a closer simulationin the form of a fresh (dead) chicken. The setup otherwise remained thesame as shown in FIG. 15. In this case, the resistance as measured oneach of the test resistive traces changed from 10.7 k to 10.25 k, andfrom 8.5 k to 9.4 k, following multiple defibrillations. During testing,it was also noted that the change in measured resistance of theresistive traces was generally consistent. It was also noted that as agiven resistive trace was increased, the (delta R {caused bydefibrillation}/R {net trace resistance}) can be minimized.

The screen printing technique for laying down resistive traces wasfurther investigated by printing a plurality of small carbon resistivedots 1601 of about 20 mm in diameter using a 7102 carbon ink applied bya screen printer (not shown). The carbon dots 1601 were laid out on atray 1602 as shown in FIG. 16A, for baking in an oven. A manuallyoperated squeegee (not shown) was used to apply the resistive dots 1601to the tray 1602 through a mask (not shown). It was determined thatcontrol of thickness during application was one important factor forcontrolling the distribution of resistance. It was noted that duringmanual application, the variation of resistance depended upon thedistance between the plurality of dots 1601 and the person applying theresistive paste, and that the pressure applied using the squeegee couldalso affect the final resistance by a factor of two. It was furthernoted that “even” heating across the tray 1602 was advantageous duringoven drying, although this factor was found to have less effect on thefinal dot resistance distribution. A resistance distribution of thebaked and measured dots 1601 is shown in the histogram of FIG. 16B. Thistesting indicated that production traces laid down on a substrate, suchas Mylar, for use in a body worn monitor should preferably be printedusing a semi-automatic screen print process, such as by a screenprinting mechanical roller process. A digital multimeter (“DMM”) withprobes placed at each edge of a dot 1601 was used to measure theresistance from one edge of the dot to the other. It was found thatuniform application of ink was important to keeping a tight distributionof resistance, with even heating having a smaller influence on thedistribution of the resistance of the test dots. Since a trace with toolow a resistance has a greater A R/R, and higher probability of failingand very high resistance traces result in worse S/N ratio, it isimportant to have a reasonably tight tolerance on the trace resistance.

FIG. 5 shows a block diagram representative of one embodiment of thebody worn physiological monitor 100. Physiological sensors 501 (such aselectrodes 109 in FIG. 1D) can be electrically coupled toelectromechanical connector 502 (as by the resistive traces 412 shown inFIG. 4). Connector 502 serves to electrically couple communications andcomputation module 102 to a disposable electrode module 110 (FIG. 2).Secondary connector 503 can also electrically couple one or moreadditional sensors, which can be situated both on and off of disposableelectrode module 110, to electromechanical connector 502 for electricalcoupling along with physiological sensor signals 501 to communicationsand computation module 102 (shown in FIG. 1) via electromechanicalconnector 505. Signals received by the communications and computationmodule 102 can be electrically coupled into communications andcomputation module 102 via electronic protection circuits 506 and/orfilters, such as ESIS filters 507.

Signals can be limited or clipped in amplitude, as needed, by protectioncircuit 506, and filtered by filter 507. One or more analog amplifiers508 can be used to amplify the amplitude limited and filtered signals.In the exemplary body worn ECG monitor, amplifiers 508 canadvantageously be differential amplifiers to amplify the differencesignal (e.g. the ECG “vector”) between two ECG electrodes. Theelectrical output of amplifiers 508 can be electrically coupled to bothPACER circuits 509 and ECG circuits 510. PACER circuits 509 aredescribed further below. ECG circuits 510 perform several functions,including “trace restore”, low pass filtering (anti-aliasing), high passfiltering, and amplification (gain). Low pass filtering filters signalsaccording to the Nyquist criterion to avoid aliasing later when thesignals are digitized by analog to digital converter (ADC) 516. The highpass filter causes the input to be AC coupled from a roll off frequencyof about 0.05 Hz, as specified by industry ECG standards. Gain isrequired to cause the small pre-amplified potentials from physiologicalsensors (such as electrodes 109) to more closely match the availabledynamic range of the digitizing ADC 516. Note that ADC 516 can be adedicated ADC chip or can be included in a microcomputer integratedcircuit, such as a microcomputer serving as microprocessor 512.

A microprocessor, such as microprocessor 512, is defined herein assynonymous and interchangeable with the terms “microcomputer”,“microcontroller”, and “microprocessor”. Such microprocessors are alsointerchangeably represented herein as “μP” or “μC”. Further, anymicroprocessor disclosed herein can be replaced by any integrated devicethat can perform the function of a microprocessor, such as, but notlimited to, a field programmable gate array (“FPGA”) programmed toperform the functions of a microprocessor.

Typically, one or more differential amplifiers can be dedicated toparticular difference voltages associated with physiological sensors 501or 504, but it should be noted that one or more amplifiers 508 can alsobe multiplexed by techniques as known in the art, to serve multiplephysiological sensors using a lesser number of amplifiers. Similarly,one or more ADCs 516 can serve two or more signals from physiologicalsensors 501 or 504 using techniques such as multiplexing in time that isdigitizing one physiological sensor difference signal at a time sendinga digital result to a next stage one after the other. ECG circuits 510and PACER circuits 509 are referred to in the plural, since there can beindividual circuits for each measured physiological signal, such as foreach measured ECG vector.

Electrical power from power source 515 can be regulated by regulator 514and distributed as regulated voltage 517 to most function blocks (asrepresented herein by the label “POWER”). Each of these function blocksalso has a control (“CTRL”) input 511 from microprocessor 512, allowingthese circuits to be disabled, when not needed, in order to save batterypower. When viewed over time, most of the ECG waveform does not containuseful information since there is significant “dead ^(time) betweenheart beats. Therefore, for example, from the end of a “T wave” at theend of one heart beat to the beginning of a “P wave” at the beginning ofthe next heartbeat, circuits can be powered down (in a device “sleepmode”) to save on the order of 60% of the energy stored in the powersource that would have otherwise been used during this dead time.

FIG. 17 shows an exemplary ECG waveform. In brief, the P wave can berelated to the electrical current causing atrial contraction. The QRScomplex can be related to ventricular contraction of the left and rightventricles. The Q wave can be related to electrical current travelingthrough the intraventricular septum, while the R and S waves can berelated to the ventricles contracting. The T-wave is due to there-polarization of the ventricles. Usually, atrial re-polarizationoccurs atop the QRS complex, and being much smaller than the QRScomplex, is not seen. The ST segment connects the QRS complex to theT-wave. Irregular or missing waves may be indicators of cardiac issuesincluding: ischemic tissue, e.g. due to myocardial infarction, bundlebranch block, atrial problems (specifically P-wave abnormalities),pericarditis, and electrolyte disturbance.

Generally, power source 515 can include one or more “button” cellstypically disposed on disposable electrode module 110; however, theblock diagram of FIG. 6 shows an embodiment of a body worn physiologicalmonitor 100 where power is supplied by a power source located on, orconnected to communications and computation module 102 instead ofresiding within disposable electrode module 110.

Beyond power saving considerations, it can also be desirable in someembodiments of body worn physiological monitor 100 to put themicrocontroller and/or other circuits, including particularly digitalcircuits, into a sleep mode during an ADC conversion cycle to minimizepickup of self generated electrical noise and to minimize power use.Preferably, the A/D circuit can acquire multiple samples and buffer thesamples, before awakening the microprocessor, which then canbatch-process the data. Buffering can be set to match the patient'sheart rate, as there is no significant clinical benefit to analyzingevery sample as it is taken.

Turning back to the input circuits, typically amplifiers 508 aredifferential or instrumentation amplifiers useful to selectively amplifydesired difference signals between connector terminals (such as an ECGvector), while rejecting common mode signals (such as interferingsignals that appear simultaneously on both connector terminals). Beyondusing a differential amplifier, other techniques can be advantageouslyused to further reduce common mode pickup (CMR) and thus to improve thecommon mode rejection ratio (CMRR) of the input amplifier stages of bodyworn physiological monitor 100. CMR is of particular concern with regardto body worn physiological monitor 100 because of the proliferation ofpotentially interfering electromagnetic fields, such as from 50 Hz or 60Hz AC power line distribution throughout a hospital. For example, manyfluorescent ceiling lamp fixtures generate strong 60 Hz alternatingcurrent (AC) electromagnetic fields that can appear as common modesignals on physiological sensors 501, such as ECG electrodes 109.

FIG. 7A shows one embodiment in which body worn physiological sensor 501comprises a plurality of ECG electrodes 109 and 701. Two electrodes 109generate an ECG difference potential for patient 703. A third electrode,reference electrode 701 can be electrically coupled to electronicscommon 704 (or other potential level) and can be used to improve CMR. Inthis embodiment, the electronics common 704 (shown as the negativeterminal of the battery in FIG. 7A) can be directly tied to the patientin the vicinity of electrodes 109. Thus, the electronics common of theelectronic circuits in communications and computation module 102 can bemade to more closely follow any change in potential in the vicinity ofelectrodes 109. Reference electrode 701 can be particularly helpful toensure that inputs 109 remain within a reasonably narrow common moderange, such as by reducing a 60 Hz potential that would otherwise appearto move the electronics common 704 at 60 Hz with respect to electrodes109.

In another embodiment as shown in FIG. 7B, virtual electrode 702performs a similar function as previously described with regard toreference electrode 701. In this embodiment, instead of creating aDC-coupled reference electrode, electrode 701 is replaced by thecapacitive coupling between flexible printed circuit layer 101 and thepatient 703, resulting in a virtual electrode 702 with an AC coupledcommon. Such AC coupling increases with decreased distance betweenflexible printed circuit layer 101 and the patient and canadvantageously reduce 60 Hz common mode signals (AC signals).

In yet another version of a directly connected electrode 701, as shownin FIG. 8A, electrode 701 can be actively driven by an electrical outputfrom communications and computation module 102. Typically, anoperational amplifier (OpAmp) or other type of amplifier can be used tocreate a “driven lead”. Driven lead circuits can be used to furtherimprove CMR over passive electrodes 701 as shown in FIG. 7A. Anexemplary circuit suitable for use to drive an electrode 701 is shown inFIG. 8A. Amplifiers (OpAmps) 810 and 811 buffer the high impedancesignals from electrode 1 and 2 (exemplary electrodes 109). Differenceamplifier 508 conveys the difference signal (such as an ECG vector) aspreviously described. The two 10 kilo ohm resistors provide an averageof the common mode signals appearing simultaneously at the inputs ofbuffer amplifiers 810 and 811. The inverting low pass filter builtaround OpAmp 812 inverts the averaged common mode pickup signal (atelectrodes 1 and 2) and applies that signal out of phase (180° phaseshifted) to a directly connected driven electrode (such as electrode701). By applying the average common mode signal to the drivenelectrode, amplifier 812 effectively suppresses common mode signals atelectrodes 1 and 2 within the effective bandwidth of the negativefeedback loop by active noise cancellation. In theory, a virtualelectrode 702 could be similarly driven, but the voltage requirements todrive a capacitively coupled common electrode are high enough to make a“driven virtual electrode” a less practical option. Thus, it can be seenthat a reference electrode can be a passive connection or an activelydriven connection.

It can also be desirable to have more than one CMR technique available.For example, in a low noise environment, a lower power referenceelectrode might be used for CMR. Then if the noise increases to a levelwhere the reference electrode provides insufficient CMR, the body wornmonitor can switch to a driven lead more suitable for CMR in a highnoise environment. In this embodiment, a particular CMR configurationcan be selected by electronic switching. FIG. 8 shows one such exemplaryswitching block represented as reference electrode switch 801.Microprocessor μP) 512 can control reference electrode switch 801 toselect direct connected electrode 701, virtual electrode 702, ordirectly connected electrode 701 additionally driven by a driven leadcircuit 802. It should also be noted that in the embodiment of FIG. 8,when virtual electrode 702 provides sufficient CMR, electrode 701 can beused as a third electrode, thus allowing body worn monitor 100 tosimultaneously measure two different heart vectors.

FIG. 9 shows one embodiment of an exemplary defibrillation protectioncircuit (506) and ESIS filter 507. As shown in FIG. 9, electrodes 109can be connected via input resistors R91 and R92. Gas discharge tubes,such as neon bulbs L1 and L2, can be used for over voltage protection byfiring at a designed voltage to prevent large potentials from appearingat the input leads to amplifier 508. The gas discharge tubes can bedisposed on either disposable electrode module 110 or on thecommunication and computation module 102. Defibrillation protectionresistors R91 and R92 can further reside in disposable electrode module110, such as in the form of the resistance of traces 412.

ESIS filters 507 can be used to satisfy AAMI standard EC13 onElectrosurgical Interference Suppression (ESIS). Standard EC13 addressesthe ability of an ECG monitor to display and process ECG signals in asatisfactory manner while connected to a patient on whom anelectrosurgical device is being used. Without such suppression, the highRF output of an electrosurgical device can render ECG monitoringimpossible and or render the monitor unusable. Resistors R93 to R98 andcapacitors C91 to C96 form cascaded low pass filter sections (e.g.R93-C1). Three cascaded single pole filters are shown on each input legof amplifier 508 as an example; more or less stages can also be used. Itis also not necessary for each section of the cascaded filter to haveidentical values or roll off points in the frequency domain to create aspecific response, e.g., Bessel, Chebychev, or other filter responseknown to those skilled in the art. Also, ESIS filters are not limited tocascaded single pole filters and can take other forms as known in theart.

Test circuit 906 can provide a relatively sharp transient signal fortesting the PACER circuit described below as part of a body worn monitor100 “power on self test “. Resistors R99 and R100 can pull the output ofthe differential amplifier 508 allowing the microcontroller (512) todetect which electrode, if any, has detached, much as a “lead failed”detection is accomplished by ECG monitors having leads. Body wornmonitor 100 does not use leads, but it is still possible for one or bothof the physiological sensors to move free of a patient's body. Suchdisconnects can occur in situations in which body worn monitor 100partially moves away from the body to which it is non-permanentlyaffixed. The input impedance at one or both of the electrodes 109changes in a sensor off (sensor disconnect) event. When a patient isattached, amplifier 508 typically has an output voltage of near zerovolts. However, if one of the electrodes 109 comes off, resistors R99 orR100 cause the output of amplifier 508 to move to a most positive output(“positive rail”) or to a most negative output (“negative rail”). Notethat the negative rail can be a small voltage near zero, in the case ofsingle supply circuit operation, and that both inputs could be pulled tothe same rail. Lead-fail detection can also be analyzed to determinewhen the device is attached to the patient and then to automaticallyenter full operational mode. Such analysis can be done at a lowfrequency.

The ESIS filter 507 also can cause a stretching in the time domain of apacer pulse so that the event is recorded by at least one sample, eventhough the pacer pulse itself is of small duration compared to the ADCsample rate and the pacer pulse is likely to occur between samples.

FIG. 10 shows an alternative circuit to accomplish over voltageprotection, such as is required during defibrillation. In FIG. 10,diodes 1001 prevent the electrode potentials from going much more thanone diode voltage drop above Vcc or below ground and resistors R91 andR92 limit current. Using circuit protection, such as gas discharge tubesL1 and L2 (FIG. 9) and/or diodes 1001 (FIG. 10) combined withresistances R91 and R92, typically in the form of resistive traces 412,a body worn device can survive multiple defibrillation cycles of atleast 360 joules.

PACER circuit 509 detects pacemaker pulses. One reason to detect apacemaker is to prevent the ECG circuitry from inadvertently registeringthe regular pulses from a pacemaker as an actual heart rhythm.Separation of a pacemaker signal from signals generated by the heart isimportant both to generate accurate ECG analysis results as well as tocorrectly detect the absence of an actual heart rhythm. For example, apacemaker continues to function even where a human heart has completelyfailed.

A pacer event (pacemaker signal) is typically a narrow pulse typicallyless than 100 microseconds wide. Because of the capacitance between thepacer in a patient and an ECG circuit, an otherwise relatively squarepacer pulse as administered at the patient's heart by a pacemaker, canappear to an ECG monitor as a pulse with a negative undershoot and anexponential return to zero that could inadvertently mimic a QRS signal.A pacer signal, however, can be recognized by an analog differentiatorand alert microprocessor 512 to the presence of a pacer and to disregardthe refractory period of the corresponding R-C recovery due to the pacersignal. The pacer detection circuit or PACER circuit can generate amicroprocessor interrupt to inform the microprocessor that a pacer eventoccurred and to mark a corresponding physiological signal in time asrelated to a pacer event. PACER circuit 509 can also cause one or morepacer related circuits to automatically power down for power saving,where it is determined that a patient is not using a pacemaker.

FIG. 11 shows an algorithm useful to determine if the pacer circuitshould be enabled. A typical PACER detection circuit uses a significantpercentage of the energy available from a power source such as batteries204. If a PACER signal is not detected, such as at power up of body wornmonitor 100, the pacer circuit can be automatically disabled allowingfor a longer battery life. Since typical PACER circuits can use severalamplifiers (OpAmps), they can consume up to one third of the analogpower, therefore securing the PACER circuits when they are not needed(i.e. the patient does not have a pace maker) can cause a significantimprovement in battery life. The algorithm also can also provide checksto determine if a demand type pace maker begins operation (which mightbe inactive at power up of body worn monitor 100) by analyzing beatvariability. While it can be advantageous to have the body worn deviceautomatically sense the presence of a pacemaker and to enable the PACERdetection circuit, the choice as to whether to enable or disable thePACER circuit can also be done by externally configuring the body worndevice. Such external configuration can be done through a hardwiredcommunication connection cable or via communications and computationmodule 102, in which communications and computation module 102 is atwo-way radio transceiver communication device capable of receiving aconfiguration command sent for a remote radio transceiver. The radiocould be 802.11 compliant, but generally would use a lighter-weight(simpler) protocol that can be more energy efficient. A suitable lighterweight protocol could be proprietary, or standards-based, such as ZigBeeor Bluetooth. A body worn physiological monitor 100 is particularly wellsuited for use in hospital environment as part of an integrated wirelessmonitoring network. The details of such monitoring networks aredisclosed in U.S. patent application Ser. No. 11/031,736 entitled,“Personal Status Physiological Monitor System and Architecture andRelated Monitoring Methods”, which is incorporated by reference hereinin its entirety.

FIG. 12 shows a high pass filter (HPF) suitable for use in ECG circuitsblock 510. An advantage of a 0.5 Hz HPF is faster recovery from DCoffsets due to patient movement, defibrillation, electrocuatery, etc.However ST segment analysis is negatively impacted if HPF cutoff isgreater than about 0.05 Hz. Thus, it is preferable to have the abilityto change between a 0.5 and 0.05 Hz cutoff frequency. The high passfilter of FIG. 12 is implemented by a low pass filter configured as aninverting amplifier in a negative feedback circuit to give a neteffective high pass transfer function from circuit input to output. Thecorner frequency of the composite filter can be adjusted by switching inresistor R2′. Alternatively, S1 can be switched at a periodic rate toplace a duty cycle on C. Note that the frequency of switching of SIshould be fast with respect to the corner frequency of the anti-aliasinglow pass filter. The graphs A-D of FIG. 13 further illustrate theperformance of the exemplary filter of FIG. 12. These graphs shownormalized amplitude on the vertical axis plotted against frequency onthe horizontal axis. FIG. 13, graph A represents a raw input signal.FIG. 13, graphs B and C are Bode Plots representing the high andlow-pass filter sections. After applying filter responses B & C to inputData A, filtered data D is the result. The HPF cutoff can be 0.5 Hz orsome lower value depending on whether R2′ is switching in or if C isduty cycled.

Another method to achieve this frequency change is to use digitalfilters implemented on Microprocessor 512 to reverse the effects of the0.5 Hz HPF, then implement a digital HPF at a lower cutoff frequency,0.05 Hz, for example. The response of the 0.5 Hz filter should be knownto implement the inverse filter. This response can be measured usingmicroprocessor 512 to trigger the test circuit 906 to create an impulse,H(s). The inverse response is the [1-H(s)] (FIG. 13, graph E) and thisinverse filter can be digitally implemented by methods familiar to thoseskilled in the art. H′(s) is the frequency response of the new HPF withlower cutoff frequency, nominally 0.05 Hz. The digital filter for H′(s)is digitally generated (F) and applied along with [1-H(s)] (FIG. 13,graph E), resulting in the frequency response displayed in FIG. 13,graph G. A high pass filter suitable for use in ECG circuits block 510can be implemented in full or in part by software that can run onmicroprocessor 512.

FIG. 14 shows how a body worn monitor 100 configured as an ECG monitorcan be situated on a patient in at least two different orientations tomeasure different heart vectors. A primary heart vector is measured byorientation from the patient's right shoulder to the left hip, as shownby position 1401. An alternative position 1402 can be more suitablewhere there is injury or where patient anatomy is such that it causesthe preferred position 1401 to be less desirable. Also, body wornmonitor 100 can be affixed to the side of patient 703 (similar to ameasurement made by a conventional ECG “V” lead) or back of a patient703 (such as where a patient needs to sleep on their stomach) to monitorstill other ECG vectors (not shown). In effect, a body worn monitor 100can be placed to pick a particular vector that can be traversed by theelectrodes 109. For example at least the first three primary heartvectors, i.e. I, II, and III, can be made conveniently available in thismanner.

While illustrated with an internal battery, it is important to note thata body worn physiological monitor 100 can be powered by either aninternal power source only, an external power source only, or by aninternal or an external power source. An internal power source can be arenewable power source, such as a rechargeable battery.

Another type of internal power source is a Peltier device operated inreverse, also called a Seebeck device. Seebeck discovered that aconductor generates a voltage when subjected to a temperature gradient.Thermoelectric couples are solid-state devices capable of generatingelectrical power from a temperature gradient, known as the Seebeckeffect. (By contrast, the Peltier effect refers to the situation whereelectrical energy is converted into a temperature gradient.) A Seebeckdevice “couple” consists of one N-type and one P-type semiconductorpellet. The temperature differential causes electron flow from hot tocold in the N-type couple and hole flow from hot to cold in the P-typecouple. To create an electromotive force (EMF), the followingconnections are made: On the cold side (i.e. the side that is exposed toroom temperature) the pellets are joined and on the hot side (i.e. thepatient side), the pellets are connected to a load, such as thecomputation and communication module 102. The open circuit voltage of aSeebeck device is given by V=SΔT, in which S is the Seebeck coefficientin volts/° K and ΔT is the temperature difference between the hot andcold sides. It is a challenge today to completely power the computationand communication module 102 from a Seebeck device that is of the samesize as the computation and communication module 102. Presently, aSeebeck device may only provide supplementary power, but as electronicsmigrate toward lower power and Seebeck coefficients and thermocoupledensities improve, a Seebeck device can be a viable long-term powersolution for a patient-worn monitor. Other methods of generating energy,such as mechanical (as is used in some wrist watches) and solar, canalso be viable methods for providing a renewable self-contained powersource for a body worn monitor.

Turning to analysis routines suitable for use on a body worn monitor,typically, ECG beat picking, such as by using wavelet or Fouriertransforms and/or matched filter analysis in the time domain can becomputationally expensive. Modeling the QRS pulse as three triangleswith alternating polarities creates a rough matched filter for the QRSpulse. Taking the second derivative results in impulse functions at thepeaks of the triangles (where the first derivative is discontinuous),and all other points are zero. The second derivative method also makesthe convolution with incoming data extremely efficient as most of themultiplies have a 0 as the multiplicand and requires minimalcomputation. The result can then be integrated twice to produce amatched-filter output, which can be fed into the beat-picking algorithmthat provides fiducial marks. Using a second matched filter that issinusoidal in shape and with appropriate discriminators, the system canprovide indications of Life Threatening Arrhythmias (LTA); that is,Asystole, Vfib, and Vtach. While the accuracy of this system is lesscompetitive with a high-end Arrhythmia solutions such as those provided,for example, by Mortara, the filters can be tuned to err toward falsepositives and upon a positive LTA response, activate transmission offull waveforms.

Research has also shown that analysis of the R-R portion of the ECGwaveform interval statistics can provide a method to predict atrialflutter. Applying this and other low-computational cost methods canallow a body worn monitor device to begin transmitting full waveformsfor either clinical or algorithmic analysis by a more powerful engine,when the probability of other arrhythmias is high. Transmitting only theR-R intervals of ECG waveforms is an example of a lossy data compressionmethod. R-R intervals comprise a string of data and the string of datacan also be compressed. Lossless or lossy data compression of the entirewaveform can be implemented to save battery life, including nottransmitting (or perhaps not even sampling) data between the T and the Pwave. Because data compression results in less data to transmit, thepower saved may offset the computation cost of the data compression.

While we have referred often to ECG applications herein, the applicationof low-intensity computational methods as a power saving measure applyequally well to other types of low power-sensors, including, but notlimited to EEG, SPO₂, temperature, and invasive or non-invasive bloodpressure measurements. Whether the body-worn medical-grade monitorperforms complex analysis or simply compares a single numeric value to asingle numeric limit, the device can function in a low-power radio stateuntil a predetermined threshold is exceeded. A body worn monitor canalso periodically send data or send data upon external request.Additionally, external devices can send commands to modify the operatingparameters and thresholds.

Turning to other communication matters, it may be that adverse eventsoccur in which no uplink is available. In a case of no uplink (failedcommunications), the body worn monitor can buffer time-stamped waveformscorresponding to any adverse events. The buffers can also storewaveforms for later analysis in which this storage is triggered by thepatient when the patient recognizes a condition, such as chest pain. Inthe case of an alarm that occurs when there is no uplink, alarms can beconfigured to be latched until confirmed by a clinician. Preferably,non-continuous data are marked (time stamp, sample number) to allowcorrelation of non-continuous data with continuous data and data arealso marked to indicate when an alarm was initiated for later dataanalysis, including algorithm performance analysis.

In those instances in which many body worn monitor devices are used inclose proximity to one another, there can be concern that the reportsfrom one body worn monitor might be interchanged with reports fromanother body worn monitor. The body worn monitor presented herein, canbe configured with a patient context (i.e. name, room number, patientID, age, etc) and can maintain that context for as long as the monitoris connected to the patient to avoid such problems. The body wornmonitor can determine the status of its connection to the patient via acontinuous vital signs monitor, pressure, temperature, galvanicresponse, or similar input. Upon detection of a loss of connection witha patient, the device can, depending upon different variable settings,either erase the patient context or when re-connected to the patient,require the care giver to confirm the patient context. When the bodyworn monitor is initially powered up or connected to a patient, it canhave a time holdoff for alarms to prevent false alarms (e.g. low heartrate, lead-fail detection) while the system stabilizes.

Regarding firmware updates, where there are large numbers of body wornmonitors in a hospital, it can be problematic to keep them all updatedwith the latest version of firmware. One solution to this problem is toprovide a wireless update ability for downloading and installing newfirmware and/or configurations into all of the body worn monitors.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawings, itwill be understood by one skilled in the art that various changes indetail may be effected therein without departing from the spirit andscope of the invention as defined by the following claims. It is furtherunderstood that several aspects of the invention, including, but notlimited to, defibrillation protection resistors, pacer detect circuitdisabling, methods for ECG signal high pass filtering, and various otherlow power modes are not limited to body worn monitors, and can be usedin ECG monitors of any type.

1. A method for providing high voltage circuit protection for a patientmonitor, the method comprising: providing a substrate that supports oneor more electrical connections to a patient's body; determining a printpattern and thickness of a first material having a first resistivity tobe printed on the substrate; determining a print pattern and thicknessof a second material having a second resistivity to be printed on thesubstrate; printing the first material onto the substrate; and printingthe second material onto the substrate wherein at least part of thesecond the material overlays the first material.
 2. The method of claim1, wherein both determining a print pattern and thickness of both thefirst and second materials comprise including a filleted section.
 3. Themethod of claim 1, wherein both determining a print pattern andthickness of both the first and second materials comprise determining aprint pattern and thickness of both the first and second materials toachieve a total resistance value.
 4. The method of claim 1, wherein bothdetermining a print pattern and thickness of both the first and secondmaterials comprise the further steps of determining a print pattern andthickness of both the first and second materials to achieve a spacingbetween two or more electrical traces of the print pattern, and applyingan insulating layer after the step of printing of the second material.5. The method of claim 1, the method further comprising: providing anECG monitor having a programmed microprocessor and a plurality ofelectrical connections to measure patient heartbeat signals; filteringthe measured heartbeat signals with an analog high pass filter having ananalog high pass cutoff frequency; removing the effects of the analoghigh pass filter with a digital inverse filter algorithm resulting insubstantially unfiltered heartbeat signals; filtering the substantiallyunfiltered heartbeat signals digitally using a digital filter having adigital high pass filter cutoff frequency lower than said analog cutofffrequency.
 6. The method of claim 5, wherein filtering the measuredheartbeat signals with an analog high pass filter comprises filteringthe measured physiological signals with an analog high pass filterhaving a 0.5 Hz analog high pass cutoff frequency.
 7. The method ofclaim 5, wherein filtering the substantially unfiltered heartbeatsignals digitally comprises filtering the substantially unfilteredheartbeat signals digitally using a digital filter having a digital 0.05Hz high pass filter cutoff frequency.
 8. An ECG monitor comprising: aplurality of electrical connections including at least one electrode,the electrical connections being couplable to a skin surface to measurepatient heartbeat signals; a power source; at least one current-limitingdefibrillation protection resistor, the at least one current-limitingdefibrillation protection resistor comprising a resistor screened onto asubstrate; and an ECG electronics module, the ECG electronics modulereceiving and processing patient heartbeat signals from the plurality ofelectrical connections, wherein the at least one current-limitingdefibrillation protection resistor is electrically disposed between atleast one of the plurality of electrical connections and the ECGelectronics module.
 9. The ECG monitor of claim 8, wherein the at leastone current-limiting defibrillation protection resistor comprises aresistor screened onto a flexible substrate.
 10. The ECG monitor ofclaim 8, wherein a mechanical interface is defined from the at least oneseries current-limiting resistor to the at least one ECG electrode, themechanical interface including a filleted edge.
 11. The ECG monitor ofclaim 8, wherein a mechanical interface is defined from the at least oneseries current-limiting resistor to the at least one ECG electrode, themechanical interface including a plurality of overlapped layers.
 12. TheECG monitor of claim 11, wherein the plurality of overlapped layerscomprise a carbon resistive layer screened over a conductive surface,the carbon resistive layer having substantially the same shape as theconductive surface.
 13. The ECG monitor of claim 8, the ECG monitorfurther comprising a pacer detect circuit to detect the presence orabsence of a pacemaker, the ECG electronics module drawing power as anelectrical load on the power source wherein the pacer detect circuit,upon detection of the absence of a pacemaker signal in a patient beingmonitored by the ECG monitor, causes the ECG electronics module todisable the pacer detect circuit in order to reduce the electrical loadon the power source.
 14. The ECG monitor of claim 13, wherein the powersource is a battery.
 15. The ECG monitor of claim 8, wherein the ECGelectronics module includes a microprocessor to process the heartbeatsignals, wherein an algorithm running on the microprocessor causes theECG monitor to enter a low power mode that disables at least one circuitof the ECG electronics module, from the end of a “T wave” at the end ofone heartbeat to the beginning of a “P wave” at the beginning of thenext heartbeat, in order to reduce said electrical load on the powersource.